Experiments are proposed to explore the relationship between structure and function of the membrane protein responsible for anion transport across the human erythrocyte membrane. This protein carries out an important biological function, and is present in the membrane in quantities suitable for detailed biochemical analysis. The activity of the protein is inhibited by extracellular proteolysis with papain. The molecular origin of the inhibition will be investigated by characterizing the products of extracellular papain digestion of the anion transport protein, and comparing them with the products produced by extracellular chymotrypsin, which cleaves the protein but does not inhibit transport. The techniques used will include preparative and analytical polyacrylamide gel electrophoresis in sodium dodecyl sulfate, and end group analysis. To facilitate these analyses, the intact protein or its major glycosylated peptide will be deglycosylated by enzymatic or chemical techniques. The kinetics of anion transport indicate that the transport step involves a conformational change between two distinct states of the protein, but the nature of this conformational change is completely unknown. The proportion of protein molecules in each of these two states may be varied by changing the Cl-gradient across the membrane. The chemical similarities and differences between these two conformations of the protein will be investigated by performing irreversible chemical modification of the protein, under conditions in which virtually all copies of the protein are in one state or the other. The chemical probes to be used include pyridoxal phosphate, 5-nitrosalicylaldehyde, FeO4, and lactoperoxidase iodination.